Commands: Added the run-emulator command.
Build command went missing.

conditions statement was missing from the command.
Therefore it was not recognized as a command by mach.
Merge pull request #6 from s-dehaan/build-command-fix

Build command went missing.
Merge pull request #5 from s-dehaan/command_run-emulator

Commands: Added the run-emulator command.
Create a place to store common conditions
Use 'mach.b2g.providers'
Rename commands to b2gcommands